Lead-zirconate-titanate (PZT) thin films are technologically important and useful due to their excellent ferroelectric and piezoelectric properties. They exhibit large piezoelectric coefficients, large polarizations and desirable dielectric permittivity. Therefore, PZT thin films are employed in a wide variety of technologically demanding applications such as in microelectromechanical systems, dynamic random access memories, and non-volatile ferroelectric random access memories.
The crystal structures and hence the ferroelectric and piezoelectric behaviours of a PZT film (i.e., Pb(ZrxTi(1-x)O3) are dependent on the stoichiometric ratio of Zr/Ti in the film (i.e., x/(1-x), hereinafter referred to as Zr/Ti ratio) (B. Jaffe, W. R. Cook and H. Jaffe, Piezoelectric Ceramics, New York, Academic, 1971). They also vary with temperature. For example, at room temperature, PZT of a low Zr/Ti ratio is tetragonal in structure while that of a high Zr/Ti ratio is orthorhombic or rhomobohedral, depending on the value of the Zr/Ti ratio. The ferroelectric and piezoelectric properties of PZT thin films are also dependent on the film texture and orientation.
Several processing techniques have been used to form PZT films, including pulsed laser deposition, chemical vapour deposition, radio frequency (RF) sputtering, sol-gel processing, and other chemistry-based processing techniques. Among these techniques, sol-gel processing provides flexibility in controlling the film texture and thickness, and is also relatively inexpensive.
In a typical sol-gel process for PZT thin films, a precursor solution is prepared and then spin-coated on a suitable substrate, such as silicon wafer or other ceramic substrates of either single crystal or polycrystal or metal substrates. The precursor film is then baked and annealed at high temperatures to form a crystallized PZT layer on the substrate. The structure and properties of the resulting PZT thin films can be varied to a certain extent by using different substrate materials. For instance, PZT film can grow epitaxially from a single crystal substrate, which often dictates the growth orientation of the PZT film. Several different dopants have also been added to PZT thin films, in order to tailor their structure and electrical properties.
The electrical properties of PZT thin films can be further improved by depositing PZT layer on a specifically designed functional layer. For example, it has been shown that depositing a PZT layer with a LaNiO3 buffer layer on metal substrates can improve dielectric properties of PZT thin film (Q. Zou, H. E. Ruda and B. G. Yacobi, Appl. Phys. Lett.; 78, 2001, p1282).
While conventional PZT films exhibit good electrical properties and can be prepared by several existing techniques, there is a need for developing novel ferroelectric/piezoelectric thin films that have improved properties and methods for forming the same.